In the recent years the exponential growth of number of mobile devices and mobile traffic, mainly driven by an increase in the demand for video services, brings new challenges for mobile network operators in terms of providing high capacity solutions with a good quality of service.
FIG. 1 is a block diagram illustrating one example of a backhaul optical network topology according to prior art.
The illustrated backhaul network PA10 comprises one optical line terminal, OLT, 10 which routes data traffic from one optical network unit, ONU, 50 to another ONU 50. The OLT is further handling the data traffic between a core network on the uplink side of the OLT and the downlink residing ONUs. Each ONU is electrically connected to a base station 80, e.g. an eNB, for serving radio telecommunication traffic to and from user equipments residing in a radio coverage area 90, i.e. cell, of the base station. The ONUs 50 are physically connected to OLT 10 via optical fibres 20, which is the transport media for the optical channels, λ and maybe a remote node. The optical channels are either directed upstream λu, from the ONUs towards the OLT, or downstream, λd.
In the 3GPP standards two interfaces are defined for eNB. One of them, which is called S1 interface, is defined for the communication between eNB and the central aggregation switch in the mobile core network, while the other one referred to as X2, is a logical interface for the direct exchange of information between base stations. In a LTE backhaul network, S1 and X2 interfaces of a LTE base station are connected to the ONU. Two ONUs may be directly connected point to point via an fiber 22 for transmitting data related to LTE X2 interfaces between two base stations 80.
An ONU is a device that transforms incoming optical signals into electronics at a customer's premises in order to provide telecommunications services over an optical fibre network. An ONU is a generic term denoting a device that terminates any one of the endpoints of a fibre to the premises network at the user side, implements a passive optical network (PON) protocol, and adapts PON Patent Data Units to subscriber service interfaces. In some contexts, an ONU implies a multiple subscriber device.
An optical line termination, also called an optical line terminal, is a device which serves as the service or network provider endpoint of an optical access network. It provides two main functions:                1. to perform conversion between the electrical signals used by the service providers equipment and the fibre optic signals used by the passive optical; network;        2. to coordinate the multiplexing between the conversion devices on the other end of that network, called either optical network terminals or optical network units.        
Coordinated multipoint, CoMP, transmission and reception is introduced in the long term evolution (LTE)-Advanced framework due to its potential for improving the network throughput and spectral efficiency especially in the cell edges. In case of CoMP, the LTE evolved nodes B, eNBs, exchange the cell information and/or user data among a cluster of adjacent nodes through mobile backhaul networks. Therefore the quality of the user signal, especially in the cell edges, is highly dependent on the CoMP backhauling solutions.
The most important barrier for the large scale implementation of CoMP is the strict latency constraint and high capacity requirements in the link between the X2 interfaces. Depending on the type of transmission techniques used for CoMP the delay requirement is ranged from less than 0.5 msec (using Common Public Radio Interface, CPRI) till 10 msec (for moderate to tight coordination). This requirement may not be easily fulfilled with the current generation of backhaul networks when considering processing delay in optical line terminal, OLT, routers and switches as well as long fibre path. The limited capacity and latency issue of the available backhaul networks might act as the bottleneck for the CoMP implementations.
Thus, when interconnecting the base station nodes, eNBs, backhauled via two or more different PONs, said interconnection has to be via either the OLT from one PON to the OLT of another PON, or from ONU to ONU directly in the different PONs. Interconnection between the OLT:s has the drawback that the strict latency constraint for the X2 interface is likely to be exceeded. For interconnection of eNB nodes belonging to different PONs, the end-to-end delay is also dependent on the distance between two OLTs, the propagation delay in the uplink direction towards aggregation network, and two times of the processing delay at the OLTs. The ONU to ONU direct interconnection between ONUs of the different PONs has the drawback that is very inflexible and expensive, especially at network involving a large number of eNBs.
Considering the long reach PON, i.e., with a reach larger than 60 km, the round trip time for the data sent from one ONU to the other ONU in the same PON, excluding the processing time is around 0.6 ms, which is already higher than delay requirements of CoMP with tight coordination. Most of the available research considered a direct point to point, PtP, fibre link between eNBs and focused on the wireless transmission issues only. However, the PtP links might not be feasible due to the high cost of fibre deployments.
Therefore, it was proposed to use a splitter box containing several splitters and Wavelength Division Multiplexing, WDM, diplexers in order to interconnect base stations directly.
FIG. 2 is a block diagram illustrating a backhaul topology according to the prior art described above.
The prior art backhaul network PA20 illustrated in FIG. 2 differs from the prior art backhaul network PA10 in FIG. 1 in that a remote node with special splitter arrangement is introduced between the ONUs 50 and the OLT 10. The remote node comprises a splitter arrangement 30 comprising several splitters. Different eNBs and their corresponding ONU:s of the same PON are interconnected in a logical point to point manner via the remote node and its splitter arrangement 30, as illustrated by the dashed-dotted lines X2:12, X2:13 and X2:23. The dashed-dotted lines illustrate the logical links between two ONUs. Thus, X2:12 illustrates the logical link between ONU:1 and ONU:2, X2:23 illustrates the logical link between ONU:2 and ONU:3, and X2:13 illustrates the logical link between ONU:1 and ONU:3. The known technique is based on the passive optical network, PON, technology and needs N+1 wavelength channels to connect N neighbours per node. Interconnection of two ONU:s belonging to different PON:s is performed via the OLT to OLT connection.
Sending back the traffic in the remote node (which is located in a distance from cells lower than 1 km in dense areas, and up to 5 km in rural areas) as well as removing any intermediate electronic processing (e.g., in the OLT, switches and routers) between X2 interfaces, will lead to a much faster virtual link than in the topology of FIG. 1 when no PtP fibre is available.
The solutions proposed in FIGS. 1 and 2 are neither scalable nor flexible. They are also technology dependent. Dynamic clustering of eNBs, typically outperforms static clustering in terms of flexibility and performance. However, the proposed networks in FIGS. 1 and 2 do not support dynamic clustering. Besides, the required splitter box in FIG. 2 may also become very complicated.